Flatworms are bilaterally symmetrical with compressed bodies. Their thickness is small. Their mouth functions to take in food and excrete wastes. They have developed nervous, muscular, reproductive, and excretory systems within the parenchyma. The cells exchange oxygen and carbon dioxide with the environment. They are flattened enough to enter body openings of other animals.

There are three classes here. Classes Trematoda and Cestoidea are parasites. Class Turbellaria are mostly free-living animals. The Order Tricladia has a digestive cavity with three major branches; their eggs are laid in tough capsules attached to solid objects; and their young are adults in miniature. Order Polycladia has numerous branches from a central digestive cavity; eggs are not shelled; and some species have planktonic larvae. Both triclads and polyclads have cilia, ocelli, a posteriorly located mouth, structures for taste and smell, and a muscular pharynx. Some are pretators, some scavengers, and some are commensals.